[GMCnet] Compression test [message #180455] |
Thu, 16 August 2012 00:16  |
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ljdavick
 Messages: 3548 Registered: March 2007 Location: Fremont, CA
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In his recent post “Help for engine” Jay Rabe noted his compression is about 150-165 psi. When I checked my compression a month ago I got 135-140. What should I expect from a 140,000 mile motor, or I suppose the real question is, what should it have been from the factory?
Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, CA
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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy
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Re: [GMCnet] Compression test [message #180465 is a reply to message #180455] |
Thu, 16 August 2012 06:58   |
Steven Ferguson
 Messages: 3447 Registered: May 2006
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IIRC, all the cylinders should be within 10%. That shows even wear and no
sealing problems.
On Wed, Aug 15, 2012 at 10:16 PM, Larry Davick <ljdavick@comcast.net> wrote:
> In his recent post “Help for engine” Jay Rabe noted his compression is
> about 150-165 psi. When I checked my compression a month ago I got
> 135-140. What should I expect from a 140,000 mile motor, or I suppose the
> real question is, what should it have been from the factory?
>
> Larry Davick
> A Mystery Machine
> 1976(ish) Palm Beach
> Fremont, CA
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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--
Fathom the hypocrisy of a nation where every citizen must prove they have
health insurance......but not everyone has to prove they're a citizen.
Steve Ferguson
Sierra Vista, AZ
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Re: [GMCnet] Compression test [message #180504 is a reply to message #180455] |
Thu, 16 August 2012 11:29   |
roy1
 Messages: 2126 Registered: July 2004 Location: Minden nevada
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ljdavick wrote on Wed, 15 August 2012 22:16 | In his recent post “Help for engine” Jay Rabe noted his compression is about 150-165 psi. When I checked my compression a month ago I got 135-140. What should I expect from a 140,000 mile motor, or I suppose the real question is, what should it have been from the factory?
Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, CA
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He probably has a different cam or higher compression pistons then you have.
Roy Keen
Minden,NV
76 X Glenbrook
[Updated on: Thu, 16 August 2012 11:30] Report message to a moderator
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Re: [GMCnet] Compression test [message #180507 is a reply to message #180455] |
Thu, 16 August 2012 11:39   |
C Boyd
 Messages: 2629 Registered: April 2006
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Sir: compression tests can vary, throttle not open, wet test (squirt of oil) will vary from dry test. Carbon build up can raise compression and also contribute to spark knock. You might run some GM top engine cleaner thru the engine per directions and then run a dry test then a wet test and see if there is any difference. Higher wet is usually leaking rings, you have good compression as is so valves should be ok..
ljdavick wrote on Thu, 16 August 2012 01:16 | In his recent post “Help for engine” Jay Rabe noted his compression is about 150-165 psi. When I checked my compression a month ago I got 135-140. What should I expect from a 140,000 mile motor, or I suppose the real question is, what should it have been from the factory?
Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, CA
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C. Boyd
76 Crestmont
East Tennessee
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Re: [GMCnet] Compression test [message #180530 is a reply to message #180507] |
Thu, 16 August 2012 14:23   |
xplorid
 Messages: 213 Registered: February 2012 Location: Boise
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Hi all - what is the target compression for my '73? When I picked up this GMCII at Jim Bounds shop in June, he told me it had even compression, but at 91. That seems absurdly low given the numbers in this post, but I drove it 3000 miles up to Ohio then back here to Idaho. Hmmmm.
1974/94 GMCII by Explorer
Manny 6.5 TD Al radiator
1 ton front 4 bags back
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Re: [GMCnet] Compression test [message #180587 is a reply to message #180530] |
Fri, 17 August 2012 00:11   |
John Sharpe
 Messages: 489 Registered: February 2006 Location: Texas
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Quote: | even compression, but at 91
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Anything below 100 is bad. I doubt that the 91 is accurate. With that you probably would not have enough power to pull the 12000# over an overpass.
John Sharpe
Humble,TX
'78 Eleganza TBI
'89 Spectrum 2000 MPI V-10
'40 Ford Panel Delivery TPI
johnasharpe@gmail.com
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Re: [GMCnet] Compression test [message #180615 is a reply to message #180587] |
Fri, 17 August 2012 08:38   |
xplorid
 Messages: 213 Registered: February 2012 Location: Boise
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Exactly - and I seem to recall getting by an overpass or two, and a few hills in TN and KY on the way up to OH. We even have some hills here in ID. So I'll use this post and get some compression testing done!
1974/94 GMCII by Explorer
Manny 6.5 TD Al radiator
1 ton front 4 bags back
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Re: [GMCnet] Compression test [message #180659 is a reply to message #180638] |
Fri, 17 August 2012 15:34  |
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ljdavick
 Messages: 3548 Registered: March 2007 Location: Fremont, CA
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This thread makes me feel better about my compression readings.
Larry Davick
Fremont, California
A Mystery Machine
'76 (ish) Palm Beach
----- Original Message -----
From: "A." <markbb1@netzero.com>
To: gmclist@temp.gmcnet.org
Sent: Friday, August 17, 2012 8:46:26 AM
Subject: Re: [GMCnet] Compression test
xplorid wrote on Fri, 17 August 2012 08:38
> Exactly - and I seem to recall getting by an overpass or two, and a few hills in TN and KY on the way up to OH. We even have some hills here in ID. So I'll use this post and get some compression testing done!
The MM for '73 and '74s says OEM compression ratio was 8.5:1
That would be about 125 PSI, I think.
The MM also says:
Minimum compression recorded in any one cylinder should not be less than 70% of the highest cylinder. For example, if the highest pressure in any one cylinder is 1501bs., the lowest allowable pressure for any other cylinder would be 105 lbs. (150 X 70% = 105).
--
'73 23' CanyonLands
UA (Upper Alabama)
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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy
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